Power semiconductors (e.g., MOSFETs, IGBTs) are useful in power electronic applications (e.g., switching-mode power supply) for switching power on/off. The on/off state of the power semiconductor typically requires a high current. As a result, a gate driver may be used between low-power electronics (e.g., a controller) and the gate of the power semiconductor so that a low power signal from the controller may control the state of the power semiconductor switch. Various embodiments of gate drivers have been used in the prior art and each has limitations.
FIG. 1 schematically depicts a gate driver using an isolated gate driver integrated circuit (IC) according to a prior art embodiment. As shown, an isolated gate driver IC receives pulse width modulated (PWM) signals from a controller and outputs signals to the gate of the power semiconductor (Q1) to switch Q1 on/off. This implementation requires a gate driver power supply and is inefficient. For example, a gate driver power supply of 2 watts (W) may be necessary to provide a required 200 milliwatts (mW) to the gate (G) of Q1. Most of the power is lost in the isolated gate driver IC and its associated output stage (i.e., power amplification) circuitry. This is due, in part, to the power consumption of the isolated gate driver IC and its associated output stage during standby (i.e., the time between switching Q1). Because the isolated gate driver IC and its associated output stage circuitry are typically in standby 90% of the time during operation, the efficiency of this embodiment is low (e.g., 10%). As a result, the circuitry used in this gate driver embodiment may be larger and more expensive than desired. Also this embodiment provides multiple channels for the harmful electromagnetic noise to propagate, making it difficult to mitigate the noise.
FIG. 2 schematically depicts an embodiment of a gate driver known as a driver with pulse transformer, or a pulse driver. The pulse driver provides improved efficiency because a power supply is used to directly power Q1. In this configuration, the power supply only provides power when Q1 is switched and not during standby. In addition, because the time spent switching (i.e., turning Q1 on/off) is typically very short, the overall power consumption of this embodiment may be low. This embodiment, however, is prone electromagnetic interference (EMI) because the transformers are designed to transfer the rising and following edge of a PWM signal, which is in the same frequency range of the EMI noise.
EMI can result from transients created by Q1 during switching and therefore is worst at frequencies at, or around, the switching transient. Because the transformers are tuned to the switching transient, transients created by Q1 during switching (i.e., transient EMI) may easily propagate through the transformer and affect operation of the low power electronics (e.g., the controller). Additionally, if the controller is coupled to multiple power semiconductors (e.g., Q1, Q2, Q3, etc.) then the negative effects of the EMI produced by one power semiconductor (e.g., Q1) may affect other power semiconductors (e.g., Q2, Q3, etc.).
FIG. 3 schematically depicts an embodiment of a gate driver known as an active gate driver. The active gate driver provides improved EMI performance but suffers from low efficiency for the same reasons as the embodiment shown in FIG. 1 (i.e., the isolated gate driver IC embodiment). The active gate driver is primarily used for its ability to control the switching of Q1 in a more precise way.
Typically, the gate of Q1 is switched by transitioning (i.e., slewing) between two gate voltages. For switching efficiency, the slew rate is typically made high; however, high slew rates can cause significant ringing in the current and voltage switched by Q1. The active gate driver is used to minimize EMI from this ringing (i.e., ringing EMI) while still providing high switching rates. To achieve this, the active gate driver includes an amplifier with programmable voltage levels (e.g., 5 voltage levels) to control the switching of Q1. In operation, the amplifier may output a switching waveform to switch Q1 quickly and without EMI from the ringing. Unfortunately, like the isolated gate driver IC embodiment of FIG. 1, the circuitry in this gate driver embodiment may be larger and more expensive than desired due to its inefficiency.
A need, therefore exists, for an isolated gate driver for a power semiconductor that is simultaneously efficient (i.e., small and inexpensive), immune to transient EMI (i.e., includes tuned filtering to block EMI), and controllable to eliminate ringing EMI (i.e., provides active gate driver capabilities).